trying to add an image to the face of an object
Assuming the dimensions of the face match the dimensions of the image (to prevent stretching) and also assuming you're using cycles render, simply 


With the cube selected, press tab to enter edit mode
Press "u" and select smart UV project. Make sure you have the faces selected.
(keep settings the same, just click "ok")
Head on over to the UV image editor
Make sure you're still in edit mode, and you will see the faces unwrapped 
on the image editor like this:
Now add your image by clicking "open" at the bottom left and find your image
Rescale the face to fit the image by selecting it (yours will have an image):

In the materials tab, click "use nodes" and on the color, click the dot next to the color and select "image texture" 
Open image
and you're done.
Here's an example :



Here is the material tab:



When you click the dot, select image texture:



If the UV unwrapping worked out, it should appear in rendered view
(note: it will not appear in solid viewport; switch to rendered or texture)